A large number of acidic, liquid or solid catalysts are known for carrying out alkylation of isoparaffins such as isobutane and/or isopentane, using olefins such as propylene, 1-butenes and 2-butenes and isobutene. Industrially, the most frequently used catalysts are concentrated sulphuric acid and hydrofluoric acid, used alone or mixed with Lewis acids such as boron trifluoride. Using hydrofluoric acid in such processes is a problem because of its toxicity and high volatility. The use of sulphuric acid in such processes causes a problem due to high consumption of the catalyst necessitating expensive re-treatment. For this reason, the use of solid catalysts or catalysts which are supported on solids such as aluminosilicates or metal oxides such as zirconia treated with sulphuric acid has been recommended. However, solid catalysts have been proved to have low selectivity and low activity. Such catalysts are of particularly low activity with low reactivity olefins such as ethylene. Further, the catalysts usually used in industry react with ethylene to form stable esters. The use of aluminium chloride has been studied and proposed.
French patent application FR-A-2 626 572 and European patent application EP-A-0 576 323 have proposed the use of liquid ionic complexes which are formed by aluminium halides with certain quaternary ammonium halides or with certain amine hydrohalides, possibly with copper, to catalyse the paraffinic alkylation reaction. Such complexes, known as "molten salts", have been described by C. H. Hussey in "Advances in Molten Salts Chemistry", vol. 5, p. 185, Elsevier, New York, 1985, and by C. A. Angell and J. W. Shuppert in J. Phys. Chem. 84, 538, 1980. Such catalysts are particularly simple to use.